RFID Tags
A Radio Frequency Identification (RFID) tag is a semiconductor chip (also referred to as a “die” or integrated circuit) that can positively respond to a wireless signal sent by a “reader” that inquires into the RFID tag's existence. By positively responding to the reader's wireless signal, the RFID tag can verify its presence to the reader. Frequently cited RFID tag applications include automated inventory management systems and automated transportation/distribution systems (e.g., a pallet affixed with an RFID tag will be able to wirelessly identify itself to an intelligent warehousing reader system so as to confirm its presence within the warehouse).
The less expensive an RFID tag solution, the easier it is to justify the expense of integrating RFID tag semiconductor chips amongst stocked or transported goods. Therefore RFID tag solutions tend to be sensitive to production and/or implementation costs. One aspect that reveals the cost sensitivity of an RFID tag solution is the integration of an RFID tag's antenna.
An RFID tag, being a wireless device, uses an antenna to receive the wireless signal sent by the reader. Generally, the larger an antenna, the more sensitive the antenna is. That is, the larger the antenna, the more apt it is to detect “weak” wireless signals such as those that might be sent by a distant reader. Therefore, in order to increase the communicative range between a reader its corresponding RFID tags, the RFID tags are each affixed with an antenna that is larger than the RFID tag die.
FIG. 1 shows a simple depiction. According to the depiction of FIG. 1, a paper antenna 101 is affixed to an RFID tag 103 with conductive glue 102. The paper antenna 101 is printed upon with conductive ink patterned in some fashion to form a working antenna. Electrical signals induced on the antenna 101 flow through the conductive glue 102 and into the RFID tag die 103. Because a paper antenna 101 is used, the RFID tag die 103 itself does not contain an “on-die” antenna.
The paper antenna 101 dramatically reduces implementation costs while allowing for an antenna size that is significantly larger than the semiconductor die 103 size. In one implementation, the paper antenna 101 is approximately a 10 mm×10 mm square and the die 103 is 1 mm×1 mm square.
Another RFID tag cost structure issue concerns the packaging of the RFID tag die. An appropriate depiction is provided in FIGS. 2a and 2b. RFID tags, being semiconductor chips, are manufactured on wafers where each wafer contains many discrete RFID tag chips. The die 208 from a same sawed wafer are typically separated by channels 210 referred to as “sawed streets”, and, a layer of tape 209 beneath the sawed wafer 202 keeps the die 208 in place. A robotic “pick-and-place” machine 201 picks individual die from the sawed wafer 202 (e.g., using a suction cup or collet 207) and places each picked die at a location 203 from where a packaging process may commence. Here, packaging can be any kind of processing that a die is subjected to after it is picked (e.g., integration into a hermetically sealed ceramic “single die” package, placement into a carrier with other die for shipment, etc.).
A typical “pick-and-place” machine generally exhibits movement 204, 205 along at least two planes to “pick-up” the die and move it to another location.
If the RFID tag chips from a same wafer are not sufficiently functionally tested so as to render a “pass” or “fail” disposition until after they have been diced from the wafer and individually packaged, the expense of packaging the portion of chips that ultimately fail their functional test is pure economic waste. Therefore it behooves the RFID tag manufacturer to eliminate this waste through some kind of functional testing that takes place prior to the packaging of the individual die.
Die Seal Rings
A die seal ring is essentially a barrier that surrounds a semiconductor die's active device area in order to block the penetration of contaminants into the die's active device area. The active device area of a die is the region of the die where most if not all of the die's transistors are located. FIGS. 3a through 3c show prior art depictions of a semiconductor die's die seal ring. FIG. 3a shows a top view, FIG. 3b shows a cross-section and FIG. 3c shows a three dimensional perspective of a basic unit of the die seal's structure.
FIG. 3a shows an RFID tag die 301 and its corresponding active device area boundary 302. A die seal ring 303 is observed surrounding the active device area between the active device area boundary and the die edge 303. FIG. 3b shows a cross section of a region 3041 of the die seal ring observed in FIG. 3a. According to the depiction of FIG. 3b, the die seal ring is observed to be a stacked structure of metal wires 3121, 3122, 3123, “bar” vias 3131, 3132 and a “bar” contact 311.
FIG. 3c shows a perspective view 314 of a continuous metal wire 312 and a bar via 313. Whereas a standard via can be viewed as a scalar “dot” of metal that runs through dielectric so as to provide vertical electrical contact between two metal wires that run along different vertical planes, as observed from FIG. 3c, a bar via 313 is “continuous wire-like” and may run the length of the wires its provides electrical contact between. A bar contact 311 is similar to a bar via with the exception that a bar contact provides electrical contact between the semiconductor substrate 310 and a metal wire (whereas a bar via provides electrical contact between vertically separated wires).
From FIGS. 3a through 3c, it is apparent that the die seal ring depicted therein is a type of stacked metal structure that essentially forms a “solid wall of metal” that surrounds the active device area. The solid wall of metal preserves the structural integrity of the active device area's features by preventing external contaminants from reaching the active device area laterally. FIGS. 3a and 3b only show three levels of metal wiring. It should be understood that die seal rings of more or less than three levels of metal wiring can be implemented depending on the type of semiconductor manufacturing process. Typically, a die seal ring will extend through each metal wiring layer used within the RFID tag's active device area.